Particulates or particulate matter from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). In order to suppress such kind of particles from being discharged to atmosphere, conventionally a particulate filter is incorporated in an exhaust pipe through which exhaust gas flows.
This kind of particulate filter is constituted by a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas passing through thin porous walls compartmentalizing the respective passages is discharged downstream.
The particulates in the exhaust gas, which are captured and accumulated on the inner thin porous compartment walls, require to be burned off so as to regenerate the particulate filter before exhaust resistance considerably increases due to clogging. However, the exhaust gas from the diesel engine in a normal operation condition rarely has a chance to obtain a temperature level at which the particulates ignite by themselves. Then, adoption of a catalytic regenerative particulate filter has been studied such as a particulate filter integrally carrying an oxidation catalyst of platinum or other precious metal or a particulate filter in combination with a separate upstream oxidation catalyst.
Adoption of such catalytic regenerative particulate filter will accelerate oxidation reaction of captured particulates to lower their ignition temperature, so that the particulates can be burned off at an exhaust temperature lower than ever before.
Meanwhile, in order to reduce NOx in the exhaust gas, there has been proposed a selective reduction catalyst incorporated in an exhaust pipe though which exhaust gas flows, said selective reduction catalyst having a feature of selectively reacting NOx with oxygen even in the presence of oxygen. A required amount of reducing agent is added upstream of the selective reduction catalyst to make reducing reaction with NOx (nitrogen oxides) in the exhaust gas on the catalyst, thereby reducing discharge concentration of NOx.
Known as this kind of selective reduction catalyst having the above-mentioned feature is, for example, a catalyst of precious metal such as platinum or palladium or of basic metal such as vanadium oxides, copper oxides or ferrioxide. However, an active temperature area of such selective reduction catalyst is generally so small that, in fact, NOx can be reduced and purified only at a part of an exhaust temperature area of a diesel engine. Thus, expansion in active temperature area and especially enhancement in low-temperature activity of the selective reduction catalyst remain large tasks to be settled.
Then, the inventors devised to arrange an oxidation catalyst upstream of a selective reduction catalyst for oxidation of NO in an exhaust gas by the oxidation catalyst into highly oxidative NO2, such highly oxidative NO2 being guided to the selective reduction catalyst for acceleration of the reducing reaction on the catalyst by the reducing agent, thereby attaining the reducing reaction even at a temperature area lower than that in usual single use of a selective reduction catalyst (see, for example, Reference 1).
[Reference 1] JP2002-161732A
Effectiveness of ammonia (NH3) used as a reducing agent for reduction and purification of NOx is well-known in a field of industrial flue gas denitration, for example, in a plant. However, in a field of automobile where safety is hard to assure as to running with ammonia itself being loaded, researches have been made nowadays on use of nontoxic urea water as a reducing agent.